1. Field of the Invention
Embodiments of the invention relate to three-phase inverter devices that convert a DC voltage into a three-phase AC voltage.
2. Description of the Related Art
DC electric power generated by solar cells, fuel cells and similar devices is typically converted into AC power by an inverter device and can either supplied directly to a load, or the converted AC power is interconnected with a system and supplied to a load. However, solar cells, fuel cells, and other power sources have characteristics such that the greater the increase in output current, the more the output voltage falls. Further, solar cells and similar have characteristics such that a decline in ambient temperature is accompanied by a rise in output voltage.
When the output voltage of a DC power source having such characteristics is converted to an AC voltage by an inverter device, in general a chopper circuit is provided between the DC power source and the inverter device. Through the action of the chopper circuit, the inverter device input voltage is held at a prescribed value, and the voltage utilization factor of the inverter device can be improved.
However, provision of a chopper circuit can be a factor impeding cost reduction of a power conversion device. Hence a method is adopted in which a chopper circuit is not provided, and the voltage of the DC power source is directly converted into an AC voltage. However, there is the problem with this method that, when the output voltage of the DC power source is high, switching losses of the inverter device increase, and the device efficiency declines.
Hence in Japanese Patent Publication No. 3759334, relating to methods of driving a three-phase electric motor, a method is proposed in which a three-phase inverter device is operated as a so-called V-connected inverter. See FIG. 8.
In FIG. 8, item 1 is a DC power source and item 2 is a capacitor series circuit, formed by connecting a capacitor 21 and a capacitor 22 in series, and is connected to both ends of the DC power source 1. The point of connection of the capacitor 21 and the DC power source 1 is a first terminal P, the point of connection of the capacitor 22 and the DC power source 1 is a second terminal N, and the point of connection of the capacitors 21 and 22 is a third terminal C. The capacitors 21 and 22 are first and second DC voltage sources which halves the voltage of the DC power source 1.
Three-phase inverter circuit 3, the input terminals on the positive side and the negative side are connected to the first terminal P and second terminal N of the capacitor series circuit. The inverter circuit 3 comprises a circuit in which positive-side semiconductor switching elements Qu to Qw connected to the first terminal P and negative-side semiconductor switching elements Qx to Qz connected to the second terminal N are connected in a three-phase bridge conFIGuration. Further, the semiconductor switching elements Qu to Qw and Qx to Qz are antiparallel-connected to the diodes Du to Dw and Dx to Dz. The connection point of semiconductor switching elements Qu and Qx, the connection point of Qv and Qy, and the connection point of Qw and Qz are respectively the AC output terminals U, V, W of the inverter circuit 3.
Switch circuit 4a is formed by connecting in parallel a mechanical switch 41 and a bidirectional switch 42. One end of the switch circuit 4a is connected to the third terminal C, and the other end is connected to the AC output terminal V. An electric motor 5a is the load of the inverter circuit 3. Control circuit 6a generates control signals for the inverter circuit 3 and switch circuit 4a. 
In the above-described three-phase inverter device, when a revolution rate instruction of the electric motor 5a is higher than a prescribed value, the inverter circuit 3 operates as an ordinary three-phase inverter. At this time, the mechanical switch 41 and bidirectional switch 42 of the switch circuit 4a are both off, and all of the three-phase semiconductor switching elements perform switching operation. As a result, most of the losses occurring in the three-phase inverter device occur in the semiconductor switching elements of the inverter circuit 3.
On the other hand, when a revolution rate instruction of the electric motor 5a is lower than a prescribed value, the inverter circuit 3 operates as a so-called V-connected inverter. At this time, the mechanical switch 41 and bidirectional switch 42 of the switch circuit 4a are on, and only the semiconductor switching elements of U phase and W phase perform switching operation.
As a result, most of the losses occurring in the three-phase inverter device occur in the semiconductor switching elements of the U phase and W phase. Hence losses of the three-phase inverter device are reduced by approximately ⅔ compared with a case in which the inverter circuit 3 is caused to perform three-phase inverter operation.
However, when a three-phase inverter device is caused to operate as a three-phase inverter, losses occur uniformly in the semiconductor switching elements of all phases. On the other hand, when the three-phase inverter device shown in FIG. 8 is caused to operate as a V-connected inverter, losses occur only in the semiconductor switching elements of the U phase and W phase. Further, losses occurring in one semiconductor switching element are greater when the device is caused to operate as a V-connected inverter than when operated as a three-phase inverter.
As a result, under the same cooling conditions, there is a problem that the output capacity is reduced for V-connected inverter operation compared with three-phase inverter operation.